Hydraulic vehicle lifts have been in use for over 30 years. Examples of lifts are tailgate type lifts and handicap lifts. Many of these lifts are designed to be "gravity-down" which means that the valving is arranged such that there is no hydraulic pump activated in the cycle to pump hydraulic fluid into or out of the hydraulic cylinder during the descent portion of the use cycle. That is, the lift may descend from the vehicle floor (transfer) level to the ground in its loaded or unloaded condition by means of gravity, and not under the hydraulic pressure produced by the pump.
In a typical use cycle, taking a handicap lift-equipped vehicle for example, the vehicle will draw up to the user's stop. The vehicle doors will open, and the lift is deployed from the interior of the vehicle. In some instances, the lift platform may be stowed vertically, either inside the vehicle or externally of the vehicle. The internal vertical stowage is the current preferred system.
The lift platform first rotates downwardly to the horizontal from its vertical stowed position. The lift platform is then at the vehicle floor level, also called the transfer level. The lift is then permitted to descend by gravity to the ground. The total time of descent from transfer level to ground, unloaded and not under pump pressure takes typically 12-15 seconds with a conventional "monarch" valve. During both the deployment from stowed to transfer level and the subsequent descent to the ground level, the lift platform is normally unloaded for the case where passengers embark (enter) the vehicle. That is, there is no load weight being carried by the platform. Once the lift platform contacts the ground and any stop barriers are released, the handicap user may roll onto the lift platform. The lift is then powered by hydraulic pressure to raise the user up to the vehicle transfer level, the user rolls off the lift into the vehicle, and the platform is then stowed (raised) into its vertical position. The user-loaded lift from ground to transfer, and vice versa, must be slow, on the order of 20-30 seconds for safety and to be sure the passenger remains calm. The vehicle doors are closed and the vehicle then proceeds on its route or trip.
The problem is that during the unloaded descent phase the lift descends very slowly, not only from the initial vertical deployment to the transfer level but also and primarily from the transfer level to the ground level since there is no weight to create a driving force on the platform. A descent of 30-40 inches, which is typical for a lift employed with a van or transit bus, takes anywhere from 10-15 seconds at 65.degree. F. for the unloaded descent, a rate on the order of 3 inches per second. In colder weather, the hydraulic oil or fluid thickens and results in noticeably slower descent times, on the order of 2 to 3 times longer.
In the case of transit vehicles, a slow, unloaded gravity-down descent constitutes a very substantial delay in boarding passengers. Since that delay occurs frequently but unpredictably during each trip, transit schedules are thrown off, and the number of trips that a given transit vehicle is capable of operating during any given shift or day is both unpredictable and substantially reduced. The only solution is to purchase additional vehicles, which may be an unaffordable expense for transit systems.
A safe speed for the loaded descent is on the order of 4-6 inches per second with 6" per second being the permitted maximum. The typical descent time for a conventional Monarch valve is 9.2 seconds at 60.degree. F. with a load of from 400-600 lbs. While the unloaded descent cycle could theoretically be speeded up by providing larger flow valves, then the loaded descent, with a handicap person loaded on the platform on his or her wheelchair, would be frighteningly fast. In addition, a faster descent, which theoretically might be acceptable for certain handicap persons, others of differing age or frailty may not be able to cope with such a rapid descent. Since the lift must be universal, that type of a theoretical solution does not appear feasible.
Accordingly, there is a vital need in the art for improvement in lift cycles having varying speeds of lift ascent and descent: controlled, slow, loaded descent under power or by gravity (no power); controlled, slow, loaded, ascent; and unloaded fast descent, particularly for mass transit uses where a piece of highly expensive equipment (the transit vehicle) becomes captive to the rate of operation of just a small part of the equipment (the lift).